The present invention relates to methods and apparatuses suited to the low temperature deposition of solid thin films of one or more elements by the technique of sequentially exposing the object being coated with chemically reactive gaseous species. It also describes a number of applications of films produced by such processes.
CVD Reactor Technology
Chemical vapor deposition (CVD) reactors have been used for decades to deposit solid thin films and typical applications are coating tools, manufacture of integrated circuits, and coating jewelry. A. Sherman, Chemical Vapor Deposition for Microelectronics, Noyes Publications, New Jersey, 1987. Up to the 1960""s many CVD reactors operated by exposing a heated object or substrate to the steady flow of a chemically reactive gas or gases at either atmospheric or reduced pressures. Since, in general, it has been desired to deposit films at as high a rate as possible as well as at as low a temperature as practical, the gases used to produce the film are extremely reactive (e.g., silane plus oxygen to deposit silicon dioxide). Then if the gases are allowed to mix for too long a time period before impinging the substrate, gas phase reactions can occur, and in extreme cases there can be gas phase nucleation and particles formed rather than deposition of continuous films. At the same time, the high rate of deposition and the reactive gases used makes it very difficult to coat large area substrates uniformly. This results in very complex and expensive commercial CVD reactors. A further complication with this method is that in some cases the films deposited do not conformally coat non-uniform surfaces. This can be particularly deleterious in the manufacture of integrated circuits.
In the 1960""s it was realized that we could lower the temperature required for thin film deposition at acceptable rates by creating a low pressure glow discharge in the reactive gas mixture. The glow discharge produces many high energy electrons that partially decompose the reactive gases, and these gas fragments (radicals) are very reactive when they impinge on a surface even at moderate temperatures. Although using a glow discharge allows lower temperature operation, commercial reactors are very complex and expensive, since uniform deposition over large area substrates is even more difficult due to the inherent nonuniformity of glow discharges and due to the added expense of complex high frequency power supplies. Also, this technique can often lead to degradation of the film conformality, due to the highly reactive nature of the radicals.
In the 1970""s atomic layer epitaxy (ALE) was developed in Finland by T. Suntola and J. Anston. U.S. Pat. No. 4,058,430 describes how they grew solid thin films on heated objects. This process involves exposing the heated surface to a first evaporated gaseous element, allowing a monolayer of the element to form on the surface, and then removing the excess by evacuating the chamber with a vacuum pump. When a layer of atoms or molecules one atom or molecule thick cover all or part of a surface; it is referred to as a monolayer. Next, a second evaporated gaseous element is introduced into the reactor chamber. The first and second elements combine to produce a solid thin compound monolayer film. Once the compound film has been formed, the excess of the second element is removed by again evacuating the chamber with the vacuum pump. The desired film thickness is built up by repeating the process cycle many (e.g., thousands) times.
An improvement to this technique was described in a later patent issuing in 1983 to T. Suntola, A. Paakala and S. Lindfors, U.S. Pat. No. 4,389,973. Their films were grown from gaseous compounds rather than evaporated elements so the process more closely resembles CVD. This was recognized to be especially advantageous when one component of the desired film is a metal with low vapor pressure, since evaporation of metals is a difficult process to control. With this approach, films were deposited by flow reactors similar to a conventional CVD reactor, where the excess of each gas is removed by flowing a purge gas through the reactor between each exposure cycle. This approach was limited to only a few films, depending on the available gaseous precursors, and all of these films were not as contamination free as desired. We will refer to this process as sequential chemical vapor deposition.
An alternative approach to operating a sequential chemical vapor deposition reactor would be to operate a non-flow vacuum system where the excess gaseous compound of each sequence is removed by vacuum pumps in a manner similar to the original Suntola 1977 process. H. Kumagai, K. Toyoda, M. Matsumoto and M. Obara, Comparative Study of Al2O3 Optical Crystalline Thin Films Grown by Vapor Combinations of Al(CH3)3/N2O and Al(CH3)3H2O2, Jpn. Appl. Phys. Vol. 32, 6137 (1993).
An early application of sequential chemical vapor deposition was for deposition of polycrystalline ZnS thin films for use in electrochromic flat panel displays. M. Leskela, Atomic Layer Epitaxy in the Growth of Polycrystalline and Amorphous Films, Acta Polytechnica Scandinvica, Chapter 195, 1990. Additional studies have shown that other commercially important solid films of different compounds, amorphous and polycrystalline, can be deposited by this technique on large area glass substrates. Among these other films are sulfides (strontium sulfide, calcium sulfide), transition metal nitrides (titanium nitride) and oxides (indium tin oxide, titanium dioxide). Elsewhere, this technique has been developed as a means of depositing epitaxial layers of group III-V (gallium indium phosphide) and group II-VI (zinc selenide) semiconductors, as an alternative to the much more expensive molecular beam epitaxy process.
To applicant""s knowledge the only literature discussing sequential chemical vapor deposition of elemental films are those that deposit elemental semiconductors in group IVB such as silicon and germanium. One such study, S. M. Bedair, Atomic Layer Epitaxy Deposition Process, J. Vac. Sci. Technol. B 12(1), 179 (1994) describes the deposition of silicon from dichlorosilane and atomic hydrogen produced by a hot tungsten filament. By operating the process at 650xc2x0 C. deposition of epitaxial films are described. Deposition of diamond, tin and lead films, in addition to silicon and germanium by an extraction/exchange method in conjunction with a sequential processing scheme similar to sequential chemical vapor deposition has also been reported M. Yoder, U.S. Pat. No. 5,225,366. Also although some of the studies reported have explored processes that may be useful at moderate temperatures, most require undesirably high substrate temperatures (300xc2x0-600xc2x0 C.) to achieve the desired sequential chemical vapor deposition growth of high quality films.
Conformal Films Deposited at Low Temperatures for Integrated Circuit Manufacture
A continuing problem in the commercial manufacture of integrated circuits is the achievement of conformal deposition of dielectric (e.g., silicon dioxide, silicon nitride) or conducting (e.g., aluminum, titanium nitride) thin solid films over large area wafers (e.g., 12 inches in diameter). A film is conformal when it exactly replicates the shape of the surface it is being deposited on.
In one paper by D. J. Ehrlich and J. Melngailis, Fast Room-Temperature Growth of SiO2 Films by Molecular-layer Dosing, Appl. Phys.Lett. 58, 2675(1991) an attempt was reported of layer by layer deposition of silicon dioxide from silicon tetrachloride and water. Although the films appear to be very conformal, there is no discussion of film quality or density, and it is likely that these films are porous making them unsuitable for thin film applications. In support of this conclusion, we can refer to a study by J. F. Fan, K. Sugioka and K. Toyoda, Low-Temperature Growth of Thin Films of Al2O3 with Trimethylaluminum and Hydrogen Peroxide, Mat. Res. Soc. Symp. Proc. 222, 327 (1991). Here, aluminum oxide deposited at 150xc2x0 C. was compared to deposition at room temperature. In this case, the room temperature films thickness reduced from 2270 xc3x85 to 1200 xc3x85 upon annealing at 150xc2x0 C. for 15 minutes confirming the high porosity of the film deposited at room temperature. Another attempt to deposit silicon dioxide by sequential chemical vapor deposition used silane and oxygen by M. Nakano, H. Sakaue, H. Kawamoto, A. Nagata and M. Hirose, Digital Chemical Vapor Deposition of SiO2, Appl. Phys. Lett. 57, 1096 (1990). Although these films, deposited at 300xc2x0 C., appeared to be of better quality, they were not perfectly conformal, and could only fill holes of an aspect ratio up to 3:1. Modem integrated circuit technology requires the ability to coat holes and trenches with aspect ratios well in excess of 3:1.
Another technologically important thin solid film that needs to be deposited with high purity and at low temperature, conformally over large area wafers, is the multilayer film of titanium and/or titanium silicide plus titanium nitride. Here, the need is for a thin titanium and/or titanium silicide layer to be deposited on a silicon contact (100 xc3x85) followed by a layer of titanium nitride (3-400 xc3x85). In a recent paper by K. Hiramatsu, H. Ohnishi, T. Takahama and K. Yamanishi, Formation of TiN Films with Low Cl Concentration by Pulsed Plasma Chemical Vapor Deposition, J. Vac. Sci. Techn. A14(3), 1037 (1996), the authors show that an alternating sequence process can deposit titanium nitride films at 200xc2x0 C. from titanium tetrachloride and hydrogen and nitrogen. However, the chlorine content of the films was 1%, and no attempt was made to deposit pure titanium metal or titanium silicide. Also, the reactor used was very similar to the conventional expensive plasma enhanced CVD reactor.
Finally, sputtered aluminum films have been widely used to fabricate integrated circuits for many years. Unfortunately, sputtering is a line of sight deposition technique, so the films tend to be non-conformal. This has become more of a problem, in recent years, as denser circuit designs have resulted in holes of high aspect ratio that need to be filled. For this reason, many attempts have been made to find a suitable chemical vapor deposition process that would be highly conformal, and several processes have been successfully demonstrated by R. A. Levy and M. L. Green, Low Pressure Chemical Vapor Deposition of Tungsten and Aluminum for VLSI Applications, J. Electrochem. Soc. Vol. 134, 37C (1987). Although conformal thin films of aluminum can be deposited by CVD, these films are still not acceptable for use in circuits, because aluminum is susceptible to electromigration and it is preferred to add several percent of copper to these films to avoid this problem. All but one attempt to carry out the CVD process with copper precursors added to the aluminum precursors have been unsuccessful. See E. Kondoh, Y. Kawano, N. Takeyasu and T. Ohta, Interconnection Formation by Doping Chemical-Vapor-Deposition Aluminum with Copper Simultaneously: Alxe2x80x94Cu CVD, J. Electrochem. Soc. Vol. 141, 3494 (1994). The problem is that although there are CVD processes for the deposition of copper, the precursors used interact with the aluminum precursors in the gas phase preventing the simultaneous deposition of aluminum and copper.
Composite Fabrication
Many schemes have been developed to fabricate composite materials, because of the unusual strength of such materials. One approach to the fabrication of such materials is to prepare a cloth preform (e.g. from threads prepared from carbon fibers), and then expose this preform to a hydrocarbon gas at high temperatures. The hydrocarbon then pyrolyses with carbon depositing on the carbon preform. Unfortunately, this process is not very conformal, so that the outer pores of the preform are sealed before the interior can be coated, and the process has to be stopped prematurely. The preform then has to be machined to remove the outer layer, and further exposure is needed. This is a slow and very expensive process which is referred to in the literature as Chemical Vapor Infiltration (CVI); see e.g., Proceedings of the Twelfth International Symposium on Chemical Vapor Deposition 1993, eds. K. F. Jensen and G. W. Cullen, Proceedings Vol. 93-2, The Electrochemical Society, Pennington, N.J.
Coating Aluminum with Aluminum Oxide
As is well known, coating aluminum with a thin layer of oxide is an excellent way to protect this material from corrosion by the elements. The traditional way of doing this is to anodize the aluminum with a wet electrochemical process (Corrosion of Aluminum and Aluminum Alloys, Vol. 13 of Metals Handbook, ASM, Metals Park, Ohio, 1989). Pinholes and other defects in the anodized layer are the source of local failure of the corrosion protection of the anodized layer. Such pinholes occur because the wet anodization process relies on the underlying aluminum as the source of the aluminum in the aluminum oxide coating, and the underlying aluminum can have many impurities and defects. A preferred approach would be to deposit the desired aluminum oxide from an external source. Although using a CVD process to carry this out is a possible choice, this has not been explored because the traditional CVD process operates at 1000xc2x0 C., and this far exceeds the melting point of the underlying aluminum.
Low Temperature Brazing
In the manufacture of high temperature, high density ceramics, there is great difficulty in fabricating unusual shapes to high accuracy. Most often the ceramic is formed in the xe2x80x9cgreenxe2x80x9d state, machined while still soft, and then fired at high temperature. After firing, the resulting high density ceramic part may require additional machining, for example, with diamond grinding wheels, to achieve the desired dimensional accuracy. In some cases, the part shape makes this additional machining difficult and expensive, and in some instances there may be no known way to reach the surface that needs to be ground. High temperature brazing of ceramic parts is an alternate technology for joining odd shapes of accurately finished ceramics. In some instances the braze metal may not be compatible with the desired application. Also the high temperature preferred for metal brazing makes it difficult to join parts of different thermal expansion coefficients. For example, it is not possible to braze aluminum to alumina ceramic, because the traditional brazing temperature would be far higher than the melting point of the aluminum.
In one embodiment, the present invention provides a reactor operated at low pressure, a pump to remove excess reactants, and a line to introduce gas into the reactor through a valve. In this embodiment, a first reactant forms a monolayer on the part to be coated, while the second reactant passes through a radical generator which partially decomposes or activates the second reactant into a gaseous radical before it impinges on the monolayer. This second reactant does not necessarily form a monolayer but is available to react with the monolayer. A pump removes the excess second reactant and reaction products completing the process cycle. The process cycle can be repeated to grow the desired thickness of film.
Because the film can be deposited one monolayer at a time, the film forming on the part tends to be conformal and have uniform thickness. The present invention may use inexpensive reactors that can coat many parts simultaneously reducing costs. For the formation of a three-element film, an additional step introduces a third reactant in the process cycle. A stable compound film of any number of elements can be formed by growing the monolayers of the elements with gaseous precursors that contain the elements. Such precursors can be halides or organometallic compounds.
It is an object of the invention to facilitate the growth of thin films of any element by using a radical generator to make available highly reactive gases (radicals).